Heap leaching

ABSTRACT

A method of leaching a metal value from a heap of a metal-containing ore and a heap leaching operation are disclosed. The method includes establishing a downward flow of a leach liquor through a section of the heap by supplying the leach liquor onto a top surface of the section and allowing the leach liquor (containing metal values in solution) to drain from a lower part of the section. The method is characterized by supplying the leach liquor onto the top surface of the section at a flow rate that is sufficient so that the downwardly flowing leach liquor saturates the section of the heap.

The present invention relates to a method of leaching a metal value froma heap of a metal-containing ore.

The present invention relates particularly to distributing leach liquorsonto top surfaces of heaps in order to optimise leaching of the heaps.

U.S. Pat. No. 6,216,962 in the name of Krieg describes heap leaching inthe following terms.

“In the commercial practice of hydrometallurgy, a bed of run-of-mine orgranular low grade ore, known as a heap, is spread over an imperviousbase or pad which may comprise sheets of plastic film, asphalt and/orcompacted clay. Crushed ore is normally heaped onto the pad to a depthto 10 to 30 feet and levelled off at the top of the heap. The ore istypically pre-crushed to a desired size.”

“After the heap has been prepared, the target metal is leached from theore by circulating the leaching solution through the heap in a processcommonly known as percolation. To effect percolation, the solution isdistributed over the top of the heap and permitted to seep down throughthe heap to the impervious pad. The impervious pad is typically slopedtoward a drain pipe or channel for recovering the metal laden solution.The liquid that is distributed over the top of the heap may be aleaching solution comprising water mixed with a leaching agent, or aleaching agent may be premixed with the ore before the ore is spreadonto the heap and relatively pure water is percolated through the heap.”

The term “leach liquor” is herein understood to mean the liquid that isdistributed over the top of a heap in a heap leaching method. Asindicated in the above-quoted passages from the Kreig US patent, theliquid may be (a) a leaching solution that includes a leaching agent andwater or (b) water—in situations where a leaching agent has beenpre-mixed with ore in a heap.

There are a number of known options for distributing a leach liquor ontoa top surface of a heap.

The options are generally based on the use of sprayer/sprinkler systemsor drip systems positioned at various locations on top surfaces ofheaps.

In some instances, the known options are relatively complicated. Forexample, U.S. Pat. No. 5,005,806 in the name of Krauth discloses a dripsystem that includes mainline pipes extending longitudinally along aside of a heap, parallel header pipes extending at spaced intervals fromthe main pipeline across the heap, and parallel, closely spaced tubesextending longitudinally between the header pipes, each tube having aplurality of outlets for leach liquor.

An object of the present invention is to provide an alternative optionfor distributing leach liquors onto top surfaces of heaps.

Generally, the known leach liquor distribution options supply low flowrates of a leach liquor onto a top surface of a heap, typically 5-15l/hr/m² of top surface.

The present invention is based on the realisation that a more effectivemethod of heap leaching is to supply higher flow rates of a leach liquoronto a heap and flood the heap.

According to the present invention there is provided a method ofleaching a metal value from a heap of a metal-containing ore, whichmethod includes:

-   -   (a) establishing a downward flow of a leach liquor through a        section of the heap by supplying the leach liquor onto a top        surface of the section and allowing the leach liquor (containing        metal values in solution) to drain from a lower part of the        section, and    -   (b) supplying the leach liquor onto the top surface of the        section at a flow rate that is sufficient so that the downwardly        flowing leach liquor saturates the section of the heap.

Preferably step (a) includes establishing a plug flow of the leachliquor through the section of the heap and step (b) includes supplyingthe leach liquor at a flow rate that maintains the plug flow of theleach liquor and maintains saturation of the section of the heap.

Preferably the plug flow is relatively high velocity plug flow.

One, although not the only, option for supplying the leach liquor instep (b) includes supplying the leach liquor as a downwardly flowingcurtain that contacts the top surface of the heap as a line or a narrowband that extends across the top surface and moving the curtain acrossthe section of the heap continuously or in a series of steps.

Preferably the narrow band is less than 1 m wide.

More preferably the narrow is less than 0.75 m wide.

Preferably the curtain is continuous across the top surface of the heap.

The requirement that the downward flow of the leach liquor through thesection of the heap “saturates” the section is understood herein to meanthat the leach liquor effectively floods the section so thatsubstantially all of the voids between ore particles in the section arefilled with downwardly flowing leach liquor.

Preferably the flow rate is greater than 15 l/hr/m² of the top surfaceof the section.

More preferably the flow rate is greater than 20 l/hr/m² of the topsurface of the section.

It is preferred particularly that the flow rate be greater than 25l/hr/m² of the top surface of the section.

In some situations it is preferred particularly that that the flow ratebe greater than 100 l/hr/m² of the top surface of the section.

It is understood herein that the term “flow rate” refers to theinstantaneous flow rate and not a time-averaged flow rate, which mighttake into account ‘rest periods’ during which no leach liquor is appliedonto the top surface of the heap.

The time period for supplying the leach liquor onto the top surface ofthe section and the flow rate of the leach liquor may be any suitabletime period and flow rate having regard to relevant factors.

Relevant factors may include the ore type, the particle sizedistribution of the ore, the mechanical properties of the ore, thecomposition of the leach liquor, and the size of the heap.

Preferably the time period is a relatively short time period comparedwith typical time periods for supplying leach liquors to heaps usingknown sprayer/sprinkler systems and drip systems.

Preferably the time period is less than 4 hours per 24 hour period.

More preferably the time period is less than 3 hours per 24 hour period.

It is preferred particularly that the time period be less than 2 hoursper 24 hour period.

Preferably the method includes supplying the leach liquor in step (b)via a distributor that can be moved over the surface of the heap.

The distributor may be of any suitable construction.

By way of example, the distributor may be a track mounted header pipewith a series of spaced-apart, spray outlets or an elongate slot outlet,and the distributor may be positioned so that the header pipe extendsacross the top surface of the heap and may be moved continuously or inthe series of steps across the section of the heap and may supply theleach liquor as a curtain onto the top surface of the heap.

Preferably the method includes positioning a barrier on the top surfaceof the heap to retain and minimise run-off of the leach liquor suppliedonto the top surface in step (b).

Preferably the top surface of the heap includes a series of furrows orother suitable troughs for retaining supplied leach liquor andminimising run-off of the leach liquor, and step (b) includes supplyingthe leach liquor into the furrows.

According to the present invention there is also provided a heapleaching operation that includes:

-   -   (a) a heap of a metal-containing ore; and    -   (b) a leach liquor distributor for supplying leach liquor onto a        top surface of a section of the heap at a flow rate that is        sufficient so that the downwardly flowing leach liquor saturates        the section of the heap.

In one embodiment, the distributor is a track mounted header pipe with aseries of spaced-apart, spray outlets or an elongate slot outlet.

Preferably the distributor is positioned so that the header pipe extendsacross the top surface of the heap and can be moved continuously or inthe series of steps across the section of the heap and can supply theleach liquor as a curtain onto the top surface of the heap.

In another, although not the only, embodiment the heap operationincludes a cover above the top surface of the heap and the distributorincludes a network of header pipes attached to the underside of thecover with a series of spaced-apart spray outlets, such that leachliquor may be applied to the top and side surfaces of the heap.

The present invention is described further by way of example withreference to the accompanying figure that is a top plan view of a heapwith a leach liquor distributor positioned on the heap.

The heap shown in the figure is a conventional construction with a topsurface 3, sloping side walls 5, sloping end walls 7, and an access ramp9 to allow vehicle access onto the top surface 3 of the heap.

Whilst not shown, the heap is set up to collect and transfer a leachliquor containing dissolved metal values from the base of the heap andto transfer the leach liquor for subsequent processing to recover themetal values (and the leach liquor), such as by solvent extraction andelectrowinning techniques.

A leach liquor distributor, generally identified by the numeral 11, ispositioned for movement along the length of the heap in the directionsindicated by the arrows on the figure.

The distributor 11 includes a header pipe 13 that is mounted on tracks17.

The figure illustrates two tracks 17 at opposite ends of the header pipe13. However, depending on the circumstances, the distributor 11 mayinclude a number of additional tracks 17 positioned at spaced intervalsalong the length of the header pipe 13 between the ends.

One end of the header pipe 13 is closed and the other end is connectedto a source (not shown) of a leach liquor and a flow control system (notshown) for supplying the leach liquor to the header pipe 13.

The header pipe 13 includes a downwardly facing slot outlet (not shown)extending along the length of the header pipe 13 that overlies the heapfor supplying the leach liquor as a downwardly flowing curtain onto thetop surface 3 of the heap.

In accordance with a preferred embodiment of the method of the presentinvention, the distributor 11 is moved so as to traverse a section ofthe heap or the whole heap (in the case of a relatively small heap) anda leach liquor is supplied onto the top surface 3 of the section of theheap via the slot outlet at a flow rate that establishes and thereaftermaintains high velocity plug flow of the leach liquor through the heapand thereby saturates the section of the heap.

Typically, the flow rate onto the top surface of the section will bemore than 15 l/hr/m² of the top surface of the section as thedistributor traverses any given section of the top surface.

Typically, the time period for supplying the leach liquor at the aboveflow rate onto the top surface of the section will be no more that 4hours in total in a 24 hour period.

Depending on the circumstances, the distributor 11 may be movedcontinuously back and forward over the section of the heap or may bemoved in steps and held at a particular location for a required timeperiod before being moved to supply the leach liquor to an adjacentstrip of the heap.

In addition, depending on the circumstances, it may be necessary toengineer the heap so that:

-   -   (a) the section of the heap can cope with high flow rates of the        leach liquor that are supplied onto the top surface of the        section of the heap in order to establish and maintain high plug        flow of the leach liquor through the section of the heap, and    -   (b) the section of the heap can cope with the required high plug        flow of the leach liquor and the consequential saturation        conditions in the heap.

In some circumstances, the above considerations may make it necessary toflood a section of the top surface of the heap to provide a reservoir ofthe leach liquor. One option to achieve this objective is to locate atemporary barrier around the section of the heap to retain a reservoirof the leach liquor. Another option, which may be used in conjunctionwith the previous option, is to form a series of furrows (not shown) orother suitable troughs in the top surface of the section of the heap toretain a reservoir of the leach liquor. Another option, which may beused in conjunction with the previous options, is to select the particlesize distribution of the ore particles in the heap to control thepermeabilty of the heap. An important consideration in relation to theseand any other options is to avoid run-off of the leach liquor from thetop surface.

The above-described method has a number of advantages over known optionsfor distributing leach liquors onto top surfaces of heaps. A majoradvantage is that the method makes it possible to increase the rate ofrecovery of metal values from an ore.

Many modifications may be made to the preferred embodiment of thepresent invention described above without departing from the spirit andscope of the present invention.

By way of example, whilst the header pipe 13 of the preferred embodimentincludes a downwardly facing slot outlet for supplying the leach liquoronto the top surface 3 of the heap, the present invention is not solimited and extends to any suitable options for supplying the leachliquor. Another option includes a series of downwardly facing sprayoutlets in the header pipe 13 for supplying overlapping sprays of theleach liquor onto the heap. Another option includes an upwardly facingslot outlet in the header pipe 13 whereby the leach liquor flows fromthe outlet and down the side wall of the header pipe 13 and downwardlyas a curtain of leach liquor.

In a situation where the heap is covered, a preferred embodiment of theinvention is to attach the leach liquor distributor to the under surfaceof the cover. In this embodiment, the distributor includes a network ofheader pipes attached to the underside of the cover with a series ofspaced-apart spray outlets, such that leach liquor may be applied to thetop and side surfaces of the heap so as to provide high flow rates toeither the entire heap or sections of the heap for any chosen timeperiod.

1. A method of leaching a metal value from a heap of a metal-containingore, which method includes the steps of: (a) establishing a downwardflow of a leach liquor through a section of the heap by supplying theleach liquor onto a top surface of the section and allowing the leachliquor (containing metal values in solution) to drain from a lower partof the section, and (b) supplying the leach liquor onto the top surfaceof the section at a flow rate that is sufficient so that the downwardlyflowing leach liquor saturates the section of the heap.
 2. The methoddefined in claim 1 wherein step (a) includes establishing a plug flow ofthe leach liquor through the section of the heap and step (b) includessupplying the leach liquor at a flow rate that maintains the plug flowof the leach liquor and maintains saturation of the section of the heap3. The method defined in claim 2 supplying the leach liquor in step (b)by supplying the leach liquor as a downwardly flowing curtain thatcontacts the top surface of the heap as a line or a narrow band thatextends across the top surface and moving the curtain along the lengthof the section of the heap continuously or in a series of steps.
 4. Themethod defined in claim 3 wherein the curtain is continuous across thetop surface of the heap.
 5. The method defined in claim 1 furtherincluding supplying the leach liquor in step (b) at a flow rate that isgreater than 15 l/hr/m² of the top surface of the section.
 6. The methoddefined in claim 5 wherein the flow rate is greater than 20 l/hr/m² ofthe top surface of the section.
 7. The method defined in claim 6 whereinthe flow rate is greater than 25 l/hr/m² of the top surface of thesection.
 8. The method defined in claim 1 wherein step (b) includessupplying the leach liquor for a relatively short time period comparedwith typical time periods for supplying leach liquors to heaps usingknown sprayer/sprinkler systems and drip systems.
 9. The method definedin claim 8 wherein the time period is less than 4 hours per 24 hourperiod.
 10. The method defined in claim 8 wherein the time period isless than 3 hours per 24 hour period.
 11. The method defined in claim 8wherein the time period is less than 2 hours per 24 hour period.
 12. Themethod defined in claim 1 further including supplying the leach liquorin step (b) via a distributor that can be moved over the surface of theheap.
 13. The method defined in claim 1 further including retaining andminimising run-off of the leach liquor supplied onto the top surface instep (b) by positioning a barrier on the top surface of the heap. 14.The method defined in claim 1 further including retaining and minimisingrun-off of the leach liquor supplied onto the top surface in step (b) byforming a series of furrows or other suitable troughs for leach liquor,and wherein step (b) includes supplying the leach liquor into thefurrows.
 15. A heap leaching operation that includes: (a) a heap of ametal-containing ore; and (b) a leach liquor distributor for supplyingleach liquor onto a top surface of a section of the heap at a flow ratethat is sufficient so that the downwardly flowing leach liquor saturatesthe section of the heap.
 16. The heap leaching operation defined inclaim 15 wherein the distributor is a track mounted header pipe with aseries of spaced-apart, spray outlets or an elongate slot outlet. 17.The heap leaching operation defined in claim 16 wherein the distributoris positioned so that the header pipe extends across the top surface ofthe heap and can be moved continuously or in the series of steps acrossthe section of the heap and can supply the leach liquor as a curtainonto the top surface of the heap.
 18. The heap leaching operationdefined in claim 15 further including a cover above the top surface ofthe heap and wherein the distributor includes a network of header pipesattached to the underside of the cover with a series of spaced-apartspray outlets, such that leach liquor may be applied to the top and sidesurfaces of the heap.